1. Field of the Invention
The present invention relates to a retrieval apparatus for searching a desired information track on a recording carrier possessing multiple information tracks, and for recording a signal on the recording carrier or reproducing a recorded signal during tracking and controlling so that the recording or reproducing position is located on the desired information track.
2. Description of the Prior Art
According to an optical recording and reproducing apparatus, as a conventional example, a light beam generated from a light source such as semiconductor laser is converged and emitted on a disc-shaped recording carrier rotating at a specific speed, and a signal is recorded on the recording carrier or the signal recorded on the recording carrier is reproduced.
On this recording carrier, tiny tracks of 0.6 micrometer in width and 1.6 micrometers in pitch are formed spirally or concentrically. When recording a signal on the recording carrier or when reproducing a signal recorded on the recording carrier, the tracking is controlled so that the light beam may be always located on the track. The error signal of this tracking control, that is, the track deviation signal expressing the positional deviation between the light beam and track on the recording carrier is obtained by receiving the reflected light or transmitted light from the recording carrier using a photosensor.
As the structure of the actuator for tracking control, for example, a two-actuator composition is known, which comprises a first actuator for moving the converging lens in order to move the light beam on the recording carrier in the widthwise direction of the track, and a second actuator for moving the first actuator in the radial direction of the recording carrier (e.g. the Japanese Laid-open Patent No. 57-147168).
As the first actuator, meanwhile, the structure of rotating a reflecting mirror to move the light beam on recording carrier in the widthwise direction of the track is known (e.g. the Japanese Laid-open Patent No. 56-153562).
In the case of tracking control by using two actuators as mentioned above, the track deviation signal is applied to the first and second actuators, and the tracking is controlled so that the light beam on the recording carrier may be always situated on the track. The relation of the first and second actuators is such that the first actuator mainly moves as for the high speed track deviation, while the second actuator is mainly responsible for the low speed track deviation.
Multiple tracks are formed on the recording carrier, and a retrieval means is indispensable for searching a track in which desired information is recorded.
Retrieval of a desired track is effected on the basis of the address given to each track on the recording carrier, and more specifically the difference between the address of the track being reproduced at the present and the address of the desired track is determined, and the second actuator is driven so that the light beam on the recording carrier may cross the tracks by the number of tracks corresponding to the obtained address difference while deactivating the tracking control, and then tracking control is started again to search the desired track (e.g. the Japan Laid-open Patent No. 54-92155).
In another known method, capable of moving the light beam on the recording carrier over a wide range, tracking control and retrieval of a desired track are realized using one actuator only (e.g. the Japanese Laid-open Patent No. 60-239943).
In the apparatus for performing tracking control by using two actuators as stated above, retrieval of desired track is effected by moving the first actuator by the second actuator, but since the transfer load changes depending on the state of installation of the apparatus at this time, retrieval may be unstable. For example, when the apparatus is installed at an inclination to the moving direction of the second actuator, the transfer load changes by the own weight. To prevent this, it is proposed to correct by measuring the inclination of the apparatus by using an acceleration sensor (e.g. the Japanese Laid-open Patent No. 60-35377). In this method, however, not only the cost is raised by using the acceleration sensor, but also the loads of friction or wire materials or their changes in the time course cannot be improved.
In other words, the friction generated when transferring the first actuator by the second actuator or the transfer load due to the wire materials for feeding current to the first actuator differs depending on the position of the first actuator. For example, the transfer load when the first actuator is positioned near the inner periphery of the recording carrier and that when positioned near the outer periphery of the recording carrier are different. As the transfer load thus varies, in the conventional apparatus, not only the discrepancy of the retrieval time increases, but also the retrieval operation becomes unstable, which may sometimes lead to the occurrence of retrieval errors or an extreme lowering of reliability. The same holds true with the apparatus composed to perform tracking control using one actuator.